M.R.I. brain scans taken before and after the participants’ meditation regimen found increased gray matter in the hippocampus, an area important for learning and memory. The images also showed a reduction of gray matter in the amygdala, a region connected to anxiety and stress. A control group that did not practice meditation showed no such changes.

Britta Hölzel, a psychologist at Massachusetts General Hospital and Harvard Medical School and the study’s lead author, said the participants practiced mindfulness meditation, a form of meditation that was introduced in the United States in the late 1970s.

“The main idea is to use different objects to focus one’s attention, and it could be a focus on sensations of breathing, or emotions or thoughts, or observing any type of body sensations,” she said. “But it’s about bringing the mind back to the here and now, as opposed to letting the mind drift.”

Generally the meditators are seated upright on a chair or the floor and in silence, although sometimes there might be a guide leading a session, Dr. Hölzel said.

Of course, it’s important to remember that the human brain is complicated. Understanding what the increased density of gray matter really means is still, well, a gray area.

“The field is very, very young, and we don’t really know enough about it yet,” Dr. Hölzel said. “I would say these are still quite preliminary findings. We see that there is something there, but we have to replicate these findings and find out what they really mean.”

Previous studies have also shown that there are structural differences between the brains of meditators and those who don’t meditate, although this new study is the first to document changes in gray matter over time through meditation.

Ultimately, Dr. Hölzel said she and her colleagues would like to demonstrate how meditation results in definitive improvements in people’s lives.

“A lot of studies find that it increases well-being, improves quality of life, but it’s always hard to determine how you can objectively test that,” she said. “Relatively little is known about the brain and the psychological mechanisms about how this is being done.”

In a 2008 study published in the journal PloS One, researchers found that when meditators heard the sounds of people suffering, they had stronger activation levels in their temporal parietal junctures, a part of the brain tied to empathy, than people who did not meditate.

“They may be more willing to help when someone suffers, and act more compassionately,” Dr. Hölzel said.